Prevent particles from intersecting with a mesh used as a container
Particles are probably not gonna cut it alone, I'm afraid. I'd go with a rigid body simulation to make it work.

Add a temporary mesh roughly shaped as the final distribution of the stones, but add it some walls to confine falling stones as below.



Turn you stones placed above said object into Rigid Body Active, and the container as Rigid Body Passive.



Adjust details as necessary, in the end remove or hide the container mesh.